The present invention relates to novel and improved heat-resistant materials and their preparation thereof. More particularly, it relates to the materials for the casting equipment of non-ferrous molten metals which have comparatively low melting points such as aluminium, titanium, zinc etc., especially in regards to a material which constitutes a portion of such equipment that is directly in contact with the molten metal, and a method for producing the same.
For low melting point metal casting equipment, a variety of heat-resistant materials have been used to constitute the above mentioned portions for carrying out such things as molten metal transfer, supply and holding, for example pouring boxes, launders and troughs, holding furnace inner linings etc., floats, spouts, hot top rings etc., and among such material the calcium silicates have been widely used due to their excellent high heat insulating characteristics, small heat capacities and further their so-called non-wetting characteristics. However, the use of asbestos fiber which has been used since the past as a reinforcing fiber for such calcium silicate type materials has become difficult to use for well-known reasons. So the production of a product not containing such fibers, but still having characteristics equivalent to those of the product of the prior art reinforced with such fibers has been desired. Likewise, the replacement of the asbestos fiber with other fibers has also become necessary for calcium silicate molded products used as building materials or heat insulations, and the replacement with alkali resistant glass fiber has almost been successful and is being put into practice in those industries. However as far as a compositional material for casting equipment for low melting point metal is concerned, because of the special requirements for the material is this field, no satisfactory solution of this matter has been in sight. That is, since crystals composing a calcium silicate type molded body contains some crystalline water depending on various crystal forms when such material is employed as casting components for low melting point metal as it is, the crystalline water is dehydrated, becoming steam, to cause various troubles in the molten metal. Accordingly, it is necessary to eliminate the crystalline water and adsorbed water by heating at about 300.degree.-600.degree. C. for about 3-24 hours, but from such heat treatment the tensile strength of the alkali resistant glass fiber deteriorates to less than 1/2 of the original. Hence, physical properties such as elasticity, toughness, strength, etc. of the final product become unsatisfactory.
In order to resolve the problem of deterioration in physical strength of the reinforcing fiber from heat treatment, the invention according to Japanese Patent Publication No. 57-49507, adopts a method of burning wherein a calcium silicate molded body is produced from slurry comprising a mixture of lime and siliceous material having a CaO/SiO.sub.2 mole ratio of 0.6-1.2, a xonotlite previously prepared by hydrothermal synthesis, a fibrous wollastonite, a reinforcing fiber and water, which is then burned. This method produces a calcium silicate molded body which has very little crystalline water from hydrothermal reaction, because the product consists largely of wollastonite as an unhydrate and xonotlite which has little crystalline water hence the subsequent heat treatment can be accomplished in a short time, with only slight deterioration of the reinforcing fiber by the heat treatment. In this production method, if the calcium silicate crystal produced from lime and silicious raw materials could be entirely low crystalline water containing xonotlite, the heat treatment could be extremely simplified or totally omitted. However, if the products of hydrothermal reaction has to be xonotlite, the autoclave condition for the molded body should be severe, hence creating a problem where the deterioration of the reinforcing fiber disturbs the exhibition of a satisfactory reinforcing effect. Consequently, a product according to this method would also have insufficient physical characteristics.
For the reasons mentioned above, if a calcium silicate with insufficient physical strength and toughness is employed as a compositional material for low melting point metal casting equipment, the biggest problem is the large cracks occurred during casting operation. Needless to say, it is easy to crack widely in a material with inferior physical characteristics and even in a material seemingly excellent in its physical characteristics due to the stress which is caused by the uneven temperature elevation from one side contact of the molten metal, and this defect could bring about serious accidents such as leakage of molten metal or equipment destruction. However, there are no operational countermeasures that can overcome the defect mentioned above.
On the other hand, simple insufficient physical strength can be overcome operationally by the use of sufficiently thick material or the use of a reinforcing back up material.